The present invention generally relates to the physical testing of rubber compounds. More particularly, the present invention relates to a process for physically testing a rubber sample, in real time, by continuously measuring the state of cure of the rubber sample and making adjustments to a test variable based upon the measured state of cure.
The physical testing of rubber compounds typically involves the measuring of tension, compression or shear. The measured parameters are generally either a resultant stress caused by an applied strain or a resultant strain caused by an applied stress. These tests seek to provide processability and vulcanization properties of a rubber sample. Processability, as defined by ASTM D1566, is xe2x80x9cthe relative ease with which raw or compounded rubber can be handled in rubber machinery.xe2x80x9d Thus, a rubber can be processed when it is able to be mixed, formed and shaped, and, thus, in order to be processed, a rubber must be capable of flow. Vulcanization, as defined by ASTM D1566, is xe2x80x9can irreversible process during which a rubber compound, through a change in its chemical structure (for example, cross linking), becomes less plastic. Thus, processability tests measure rubber properties while the rubber sample is still plastic and able to flow, and vulcanization tests measure rubber properties as the rubber sample changes from a plastic state to a more rigid state.
The rubber samples tested typically contain vulcanizing agents. Thus, the rubber samples will vulcanize to some degree during testing, and may purposefully be caused to vulcanize during testing, such that both processability and vulcanization characteristics may be analyzed in a single test, using appropriate equipment and test conditions. Particularly, the vulcanizing agents within the rubber sample will cause it to vulcanize during the test, and the vulcanization of the sample may be accelerated by increasing the temperature of the sample. Upon completion of vulcanization (i.e., full cure), the temperature of the rubber sample may be decreased and the after cure properties of the rubber may be analyzed. ASTM D6601 addresses the concept of curing a vulcanizable material at an elevated temperature, and subsequently reducing the temperature to measure after-cure properties. Thus, effective test equipment and methods measure processability characteristics of a rubber sample while it is able to flow, measure vulcanization characteristics of the rubber sample from scorch to full cure, and thereafter measure after-cure characteristics of the rubber sample.
Many instruments have been devised to test rubber samples in order to quantify processability and vulcanization properties of the rubber. Devices of the prior art include plastometers, curemeters, viscometers and rheometers, such as those disclosed in U.S. Pat. Nos. 2,037,529; 3,182,494; 3,479,858; 3,488,992; 3,494,172; 3,688,568; 4,829,830; 4,953,406; and 5,526,693, all of which are incorporated herein by reference. The earlier prior art devices were generally operated at isothermal conditions. It was realized that isothermal conditions did not simulate actual factory conditions, and U.S. Pat. No. 3,531,996, incorporated herein by reference, taught a method for testing the rubber sample at variable temperatures specified by a predetermined time/temperature profile, and, it is now common practice in the art to employ such time/temperature profiles. Thus, present day equipment and methods, as mentioned above, offer the ability to ramp temperatures from one given temperature to another given temperature at a specified time, and non-limiting examples of these, all incorporated by reference, are reported by John Sezna, Paper #173, presented to the Rubber Division, ASC, Orlando, Fla. 1999; John Sezna, Paper presented to the Northeast Ohio Rubber Group, Apr. 17, 2001; H. G. Burhin, Rubber Technology International, pp. 41-43 (1997); and DIN 53 529, Part II, Deutsches Institute fur Normung (1983).
It has also been found to be beneficial in measuring processability, vulcanization and after-cure characteristics to vary test parameters other than temperature. For instance, ASTM D6204 suggests measuring flow properties according to (1) a frequency sweep in which the frequency is programmed to change in steps under constant strain, amplitude and temperature; (2) a strain sweep in which the strain amplitude is programmed to change in steps under constant frequency and temperature conditions; and (3) a temperature sweep in which the temperature is programmed to either increase or decrease under constant strain, amplitude and frequency. U.S. Pat. No. 4,552,025, incorporated herein by reference, also describes that it is desirable to measure the viscoelastic properties of a rubber sample at one or more frequencies and temperatures, in order to better quantify the material""s processability.
From the above, it can be seen that the processes and devices employed in determining rubber characteristics should be multifunctional, being capable of determining processability characteristics at one or more temperatures; vulcanization characteristics at the same or other temperatures; and after-cure properties at the same or other temperatures. Also, these various characteristics may be analyzed with variable conditions other than temperature, as mentioned above, wherein it was noted that the frequency or amplitude of an applied strain may be the variable condition. In addition, it is desirable that present methods and devices be capable of defining the physical properties of complex dynamic torque, elastic torque, viscous torque, complex dynamic modulus, elastic modulus, viscous modulus, and tangent delta, as defined in ASTM D5289, ASTM D6204, and ASTM D6601.
Processability characteristics testing takes place before the onset of vulcanization (i.e., before scorching of the rubber sample), vulcanization characteristics testing takes place from the time of scorching of the sample until full cure thereof, and after-cure characteristics testing takes place after full cure. The focus in the art is to either step or ramp change a test condition for one or more of these test periods. Currently, changing a test condition during rubber sample testing requires specifying test conditions prior to the start of the test, such that the variable test condition is step changed or ramped at a predetermined time that may or may not closely approximate the actual scorch time or full cure time. Because the times necessary for reaching scorch and full cure may vary from one rubber sample to another, researchers typically err on the side of making test times unnecessarily long in order to ensure that (1) all sample batches scorch before the condition is varied, in vulcanization testing, and (2) all sample batches are completely cured before the condition is varied, in after cure testing.
In general, a trial-and-error approach is employed to determine a time/variable condition profile (i.e., a profile of the time(s) at which the variable condition should be step changed or ramped) for vulcanization testing and/or after cure testing, in order for a set of rubber batches to be tested and compared. For example, in the case of using a curemeter to discriminate scorch differences between batches, a test must be run until it scorches, and, in order to be assured that scorch occurs, the test time must be long enough so that all batches scorch within the predetermined time. By design, this time is typically unnecessarily long for some of the batches. Likewise, when fully curing rubber batches, for a fixed time, for the purpose of analyzing after cure properties, the time must be long enough to ensure complete cure for all batches, as under cure or over cure will affect the after cure properties. Thus, tests for analyzing after cure properties are also typically unnecessarily long in order to ensure that full cure has been reached for all batches being tested. In the prior art, a good, discriminatory time/variable condition profile is determined only after running multiple experimental profiles with particular rubber batches and becoming familiar with them in order to determine if a particular time/variable condition profile correlates with the scorch and the full cure of the rubber batches. This general trial-and-error approach to determining test conditions, in addition to being burdensome and time consuming, may be inaccurate because differences between rubber batches might be such that a determined time/variable condition profile would call for changing the variable condition either too early or too late.
Thus, there is a need in the art for a method for testing the properties of a rubber sample wherein a test condition is varied, during testing, not according to experimentally predetermined time/variable condition profiles, but rather, according to the actual measured state of cure of the rubber sample itself. The need also exists for a method wherein each rubber batch variation is treated uniquely, according to the unique physical characteristics of that batch, during the vulcanization thereof. In particular, there exists a need in the art for a method for testing the properties of a rubber sample wherein the variable test condition is changed upon the scorch of the rubber sample being tested, and is additionally, or in the alternative, changed upon full cure of the rubber sample, wherein the time in which the variable test condition is changed is unique to each batch, thus saving time over the prior art where test conditions are not changed according to the characteristics of each individual rubber sample.
As used herein, xe2x80x9crubberxe2x80x9d or xe2x80x9crubber samplexe2x80x9d is to be understood to mean any vulcanizable polymeric material suitable for testing according to the method disclosed herein. These polymeric materials may include, by way of non-limiting example, elastomers, elastomeric compounds, thermoplastic elastomers, thermoset plastics, and the like. The types of polymeric materials suitable for testing according to this invention will be readily known to those of ordinary skill in the art.
As used herein, xe2x80x9cstate of curexe2x80x9d refers to the degree to which the rubber sample has been vulcanized or cured. The state of cure of a rubber sample can be generally quantified with reference to a measured physical property of the rubber sample, as, for example, with reference to measurements of torque or modulus. The state of cure of a rubber sample lies along the continuum from a completely uncured state to a completely cured state, and information regarding the state of cure of a given rubber sample can be obtained by analyzing a physical property of the rubber sample during the curing thereof.
In general, the present invention provides a process for testing the properties of a rubber sample wherein the rubber sample is subjected to a changing test condition. The process includes the steps of continuously measuring at least one physical property of the rubber sample to analyze the state of cure of the rubber sample; and adjusting the test condition when the state of cure is analyzed to satisfy a predetermined threshold.
In a more particular embodiment, this invention provides a process for testing the properties of a rubber sample comprising the steps of: contacting the rubber sample with a rotating or oscillating test instrument element; continuously measuring, as a function of time, a physical property of the rubber sample by analyzing the response of the rubber sample to the rotating or oscillating test instrument element, wherein the physical property is indicative of the state of cure of the rubber sample; setting a threshold for one or both states of cure selected from: (a) scorch of the rubber sample and (b) full cure of the rubber sample, wherein the threshold, whether for state (a) or (b), is based either upon a magnitude of the physical property measure in said step of continuously measuring or upon a calculated slope of the physical property as a function of time; and changing a test condition to which the rubber sample is subjected, when the threshold is found to be satisfied by the physical property measured in said step of continuously measuring, wherein the test condition is selected from temperature, degree of strain, frequency of oscillation, and combinations thereof.